Decide: should `estimate <plan.json>` price the full post-apply fleet or only changed resources?

[andreacappelletti97]

Splitting this out of the PR #46 review so the decision is explicit before merge.

Background

Today's plan parser reads only resource_changes and filters no-op actions, so c3x estimate plan.json returns roughly the cost of what this apply changes (near-$0 for a plan where most resources are unchanged). #46 rewrites it to walk planned_values (the full post-apply resource set), so estimate plan.json returns the total cost of all resulting infrastructure.

Why it needs a decision

Changing the default output of estimate plan.json is a behavior change for existing users:

• --budget gates can flip: a plan that added one $10/mo resource used to total ~$10; now it totals the whole fleet (hundreds/mo), which may fail a CI budget that previously passed.
• --format json payload changes shape (all resources + full-fleet project_total instead of the changed subset).
• The --show-delta flag added in Improve plan parser completeness and support optional() type defaults #46 gives the changed-only view on demand.

Options

1. Adopt full-fleet as the default (what Improve plan parser completeness and support optional() type defaults #46 does). Rationale: estimate should report the cost of the infrastructure the source describes; the old changed-only total was arguably misleadingly low. Use --show-delta for the change view. Requires a clear CHANGELOG/README note and a heads-up for --budget users.
2. Keep changed-only as the default, make full-fleet opt-in.
3. Add a flag to select the mode explicitly.

Recommendation

Option 1, with an explicit CHANGELOG entry and a note in the estimate docs, since it matches the meaning of estimate elsewhere in the tool. Tracking here so it's a conscious call rather than an implicit side effect of #46.

Refs #46.

[andreacappelletti97]

Decided: Option 1 — full post-apply total. Implemented in #52 (merged).

Rationale: it's consistent with c3x's own .tf behavior; the old changed-only plan total was an inconsistency, not a feature. Plans without planned_values fall back to resource_changes (keeping unchanged, excluding delete-only).

--budget now gates the full projected total (documented in estimate --help and the release notes). The change-only view stays as follow-up via c3x diff and the --show-delta work in #46.